I50 



KEEWATIN AND LOWER HURONIAN 



This oldest group is a highly metamorphosed complex 

 consisting of a variety of acid and basic eruptives and 

 minor amounts of banded iron formation (Keewatin) ; and 

 a younger sedimentary series of conglomerate, greywacke, 

 slate with local iron formation phases, and associated 

 eruptives (Lower Huronian). These hav ; endured at 

 least one orogenic disturbance which folded them and con- 

 verted them into highly inclined schists. Granite batho- 

 liths, which were intruded at probably the same period, 

 have further metamorphosed them near their contacts to 

 crystalline, hornblende gneisses, amphibolites, mica schists, 

 etc. But, away from the batholithic contacts, where 

 metamorphism has been less intense, the ellipsoidal, 

 amygdaloidal, and other structures of the Keewatin 

 volcanics and the stratification of the Huronian sediments 

 are still sometimes recognizable. An unconformity exists 

 between the Keewatin and Lower Huronian, but so pro- 

 foundly has it been modified by these metamorphic changes 

 that its importance can not be satisfactorily estimated; 

 in fact it is usually difficult to recognize. In later Pre- 

 Cambrian time, protracted erosion unroofed the granite 

 batholiths and removed all except the synclinal portions of 

 this folded Keewatin-Lower Huronian group leaving it 

 distributed much as it is today [7 and 23]. 



LAURENTIAN. 



This term is applied broadly to all granites and gneisses 

 in the region. As indicated in the diagram, the Keewatin- 

 Lower Huronian areas form a rude meshwork, the inter- 

 spaces of which are occupied by more or less distinct, oval 

 aieas of gneiss and granite, that probably represent indi- 

 vidual batholithic domes. These rocks are dominantly 

 granodiorites, though granites and syenites are also 

 present. A gneissic texture is more common than a gra- 

 nitic one, particularly in granodiorite facies. Also, near 

 contacts with the Keewatin-Lower Huronian complex, 

 the gneiss contains a varying quantity of these older 

 materials which have been stoped off and floated out into 

 the granite magma while the latter was still plastic. The 

 distribution and character of these inclusions would imply 

 that the gneissic structure of the enclosing rock also was 



